[5.4] Check Htmlable contract instead of instanceof HtmlString in Mailer (#18459)

* Check Htmlable contract instead of instanceof HtmlString in Mailer

* Update Mailer.php

Author: jbraud

src/Illuminate/Mail/Mailer.php

@@ -6,8 +6,8 @@
 use Swift_Message;
 use Illuminate\Support\Arr;
 use InvalidArgumentException;
-use Illuminate\Support\HtmlString;
 use Illuminate\Contracts\View\Factory;
+use Illuminate\Contracts\Support\Htmlable;
 use Illuminate\Contracts\Events\Dispatcher;
 use Illuminate\Contracts\Queue\ShouldQueue;
 use Illuminate\Contracts\Mail\Mailer as MailerContract;
@@ -294,7 +294,7 @@ protected function addContent($message, $view, $plain, $raw, $data)
      */
     protected function renderView($view, $data)
     {
-        return $view instanceof HtmlString
+        return $view instanceof Htmlable
                         ? $view->toHtml()
                         : $this->views->make($view, $data)->render();
     }